Session R38: Focus Session: Scalable Technologies for Photovoltaics I

Increasing the band gap of iron pyrite by alloying with oxygen

Systematic density functional theory studies and model analyses have been used to show that the band gap of iron pyrite (FeS$_{2})$ can be increased from $\sim$ 1.0 to 1.2 -1.3 eV by replacing $\sim$ 10{\%} of the sulfur atoms with oxygen atoms (i.e., $\sim$ 10{\%} O$_{S}$ impurities). O$_{S}$ formation is exothermic, and the oxygen atoms tend to avoid O-O dimerization, which favors the structural stability of homogeneous FeS$_{2-x}$O$_{x}$ alloys and frustrates phase separation into FeS$_{2}$ and iron oxides. With an ideal band gap, absence of O$_{S}$induced gap states, high optical absorptivity, and low electron effective mass, FeS$_{2-x}$O$_{x}$ alloys are promising for the development of pyrite-based heterojunction solar cells that feature large photovoltages and high device efficiencies. Acknowledgement: We thank the NSF SOLAR Program (Award CHE-1035218) and the UCI School of Physical Sciences Center for Solar Energy for support of this work. Calculations were performed on parallel computers at NERSC and at NSF supercomputer centers.   

Effect of annealing conditions on the healing of sulfur vacancy in pyrite FeS2(100) surfaces

Through density functional calculations, we investigated the segregation of a sulfur vacancy from interior sites outward to the FeS$_2$(100) surfaces in different surface conditions in order to provide guidance for the development of iron pyrite in photovoltaics applications. We found that the surfaces with interior S-vacancy are energetically unstable and bulk S-vacancies tend to hop toward the surface, in particular when the surface composition is in the stoichiometric or S-rich side. The segregation process is accompanied by redox reaction near the vacancy site, Fe(2$+) \quad +$ S(1-) $\to $ Fe(3$+) \quad +$ S(2-), and the activation energy decreases near the surface region. We compare the calculated structural, energetic and electronic properties to experimental data, and provide insights for reduction of vacancy density in optimal annealing conditions.   

A new paradigm for thin-film solar cells: the case of Earth abundant Cu-N ternary compounds

The design of thin film solar cells is extremely sensitive to the choice of the material forming the absorbing layer. Indeed, many of the limitations of solar cell devices are either directly linked to the intrinsic properties of the absorber such as in CdTe or design-related indirect consequences of this choice such as for SnS-based devices. Most of the design of current thin film solar cells rely on chalcogenide materials as the absorbing layer. We propose a new paradigm based on Earth abundant Cu-N ternary compounds as the absorbing layer. We will present the theoretical and experimental investigation of the electronic properties of two Cu-N compounds with interesting photovoltaic properties namely CuSrN and CuTaN$_{2}$. We performed state-of-the-art defect calculation and GW-based band structure calculations. CuTaN$_{2}$ was synthesized by ion exchange and its absorption onset was subsequently characterized with diffusive reflectance. While CuSrN displays interesting p-doped capability and defect immunity similar to Cu(In,Ga)Se$_{2}$, CuTaN$_{2}$ presents very strong absorption with a sharp absorption onset in the optimal range for photovoltaic conversion. Finally, we will address potential pitfalls of such absorbers related to stability with respect to O$_{2}$ and H$_{2}$O.   

PLD growth of thin film Zinc Phosphide

The development of efficient, low cost solar cells to meet society's growing energy needs has triggered tremendous interest in developing photovoltaics formed from earth abundant materials. Zinc phosphide (Zn3P2) is a promising earth abundant absorber layer for photovoltaic energy conversion with a nearly ideal band gap (1.5eV) and a large absorption coefficient of 10$^{4}$/cm. In this work we examine the growth parameters, electrical and optical properties of thin film zinc phosphide produced using pulsed laser deposition (PLD) from a zinc phosphide target at laser fluencies ranging from 1-3 J/cm2. For the laser fluences explored, highly resistive amorphous zinc phosphide thin films were produced with a band gap of approximately 1.7 eV. The thin films could be transformed from amorphous to polycrystalline zinc phosphide by annealing at 400C for 15mins in a N2 atmosphere. High resolution X-ray photoelectron spectroscopy (XPS) is used to examine the binding energies of Zn 2p3/2 and Phosphorous 2p3/2 signals and are in the range of 1021.6 eV and 127.5 eV. Energy Dispersive X-ray Spectroscopy (EDAX) revealed that the Zn3P2 thin films are nearly stoichiometric in composition. Hall mobility in these materials and Zn3P2/ZnS hetrojunction solar cell performance will be discussed.   

The electronic properties of point defects in earth-abundant photovoltaic material Zn$_{3}$P$_{2}$: A hybrid functional method study

Zinc phosphide (Zn$_{3}$P$_{2})$ is an attractive and promising semiconductor for thin-film solar cell application because of its earth abundance and ease of thin-film fabrication. The electronic properties of intrinsic and extrinsic defects in Zn$_{3}$P$_{2}$ are studied by density-functional theory with hybrid functional method. Our results show that undoped Zn$_{3}$P$_{2}$ should be intrinsically $p$-type with Zn vacancies as the responsible shallow acceptors. Na or Cu doping is expected to result in improved $p$-type conductivity as compared to intrinsic Zn$_{3}$P$_{2}$. S or Al doping may lead to weak $n$-type Zn$_{3}$P$_{2}$. Doping of Mg does not produce good $n$-type Zn$_{3}$P$_{2}$, consistent with experimental observations. Contradicting to conventional wisdom, an interstitial P in Zn$_{3}$P$_{2}$ is not a triple-hole acceptor and a P vacancy in Zn$_{3}$P$_{2}$ is not a triple-electron donor. Instead, we find that the interstitial P is actually a single-hole acceptor and the P vacancy is a single-electron donor. The origins of these unusual behaviors are discussed.   

Local structure of Cu2S/ZnS multi-layer films prepared using ALD

We present local structure studies of ZnS, Cu$_2$S, and ZnS/Cu$_2$S composite films, using extended x-ray absorption fine structure (EXAFS) technique. The films were prepared using atomic layer deposition (ALD), which can in principle deposit films layer by layer and hence form mesoscopic structures. ZnS and Cu$_2$S films prepared using ALD are very similar to the bulk material; the main difference is a reduced amplitude for the second neighbor Zn-Zn peak in ZnS, suggesting increased disorder within the film. Relative disorder in the films also increases with decreasing thickness as well as with decreasing deposition temperature. More importantly, multi-layer ZnS/Cu$_2$S films prepared using the same parameters as for individual films do not produce the expected multi-layer for $\sim$1 nm thick layers. If there is some excess Zn, the multi-layer is predominately ZnS and the Cu$_x$S fraction is highly disordered, and may include some ZnS:Cu. In contrast if there is a little Cu excess, the film is nearly all Cu$_2$S and the small Zn fraction is highly disordered ZnS with a shifted Zn-S distance. Consequences for multi-layer formation for solar cell applications will be discussed.   

Enhancement of solar absorption with black Cu2O Nanostructures

Cu$_{2}$O is a direct gap semiconductor with a band gap of 2.1 eV. It was considered to be a solar absorber material, while the application is hindered by its large band gap and weak stability. Here we report an electrochemical synthesis of Cu$_{2}$O. By rationally control the synthetic parameters, we achieved two types of Cu$_{2}$O: one of black color and the other ``normal'' red Cu$_{2}$O. Both Cu$_{2}$O films were in cubic phase and their crystal structures are almost identical as seen by X-ray diffraction. This is further corroborated by their nearly identical Raman spectra. The scanning tunneling spectrum (STS) revealed a gap in the red Cu$_{2}$O around 2.1 eV and a significantly lowered gap of $\sim$ 1.7 eV in the black Cu$_{2}$O, indicating that the black color is caused by a change in the electronic structure. The reflectance and transmittance indicated a band gap of $\sim$ 1.7 eV for the black Cu$_{2}$O, with a significantly broadened absorption spectrum. While further effort is needed to understand the mechanism for the lowering of the band gap, we believe that our approach demonstrated means to promote earth abundant and nontoxic materials for potential photovoltaic applications through band gap engineering.   

First-principles electronic structure of $\beta $-FeSi$_{2}$ and FeS$_{2}$ surfaces

Applying density functional theory in the framework of the full-potential linearized augmented plane-wave (FLAPW) method [1], we investigated electronic structure of potential future photovoltaic materials, $\beta $-FeSi$_{2}$ and FeS$_{2}$, for selected surface orientations and terminations. The most stable orientations are determined by comparing their surface energy. Detailed electronic structure calculations show that surface states originating from Fe play an important role and might determine photovoltaic properties. Our results show that anti-ferrimagnetic ordering exists for Fe-terminated surface. Furthermore, we also studied how electronic structure and photovoltaic efficiency are affected by the recently observed structural defects such as stacking fault in $\beta $-FeSi$_{2}$. \\[4pt] [1] www.flapw.de   

Photocurrent studies on continuous large area monolayers of WS$_2$ and MoS$_2$

Continuous large area monolayers of WS$_{2}$ and MoS$_{2}$ synthesized by chemical vapor deposition were used as light sensing devices. I-V measurements and photo response measurements were performed on both materials. The photocurrent measurements were carried out from 300 $^{\circ}$K down to 10 $^{\circ}$K using various visible laser wavelengths (405 nm, 488 nm, 514 nm and 667 nm). A resistance decrease was registered on both materials when illuminated with the laser beam, such change was proportional to the laser photon energy and when the laser energy was lower than the band gap of each material, no photo response was observed. The layered materials were structurally characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. Raman spectra confirms the presence of monolayers and UV-visible spectra revealed the resonance peaks at the energies close to the direct band gap predicted for single layers of WS$_{2}$ and MoS$_{2}$ (2.05 eV and 1.85 eV ). Further experiments on time response and continuous spectral response are now underway and will be presented.   

Printable CIGS thin film solar cells

Among the various thin film solar cells in the market, CuInGaSe thin film solar cells have been considered as the most promising alternatives to crystalline silicon solar cells because of their high photo-electricity conversion efficiency, reliability, and stability. However, many fabrication methods of CIGS thin film are based on vacuum processes such as evaporation and sputtering techniques which are not cost efficient. This work develops a solution method using paste or ink liquid spin-coated on glass that would be competitive to conventional ways in terms of cost effective, non-vacuum needed, and quick processing. A mixture precursor was prepared by dissolving appropriate amounts of composition chemicals. After the mixture solution was cooled, a viscous paste was prepared and ready for spin-coating process. A slight bluish CIG thin film on substrate was then put in a tube furnace with evaporation of metal Se followed by depositing CdS layer and ZnO nanoparticle thin film coating to complete a solar cell fabrication. Structure, absorption spectrum, and photo-electricity conversion efficiency for the as-grown CIGS thin film solar cell are under study.   

Crystal structure of Cu$_2$Te predicted within adaptive genetic algorithm

Cu$_2$Te is one of the most commonly used conductive back-contacting materials for high-efficiency CdTe-based solar cells. However, the detailed crystal structure of Cu$_2$Te is still undetermined blocking property investigations of the Cu$_2$Te-based solar cell systems. Some models have been proposed but all of them have positive formation energy [1]. We have performed adaptive genetic algorithm crystal structure search to find low energy crystal structures of Cu$_2$Te. We found a new layered-structure edged by Te atoms with negative formation energy from first-principles calculations within local density approximation. This layered structure consists of tilted Cu$_2$Te ribbon arrays. Structural and electronic properties of the newly found Cu$_2$Te structure will be discussed in detail.\\[4pt] [1] J. L. F. Da Silva, S.-H. Wei, J. Zhou, and X. Wu, Appl. Phys. Lett. 91, 091902 (2007)   

High photoactivity in ultrathin as-grown hematite films prepared by atomic layer deposition

Nanostructured hematite ($\alpha$-Fe$_2$O$_3$) has been widely studied for use in a variety of thin film applications including solar energy conversion, water oxidation, catalysis, and gas sensing. Among established deposition methods, atomic layer deposition (ALD) is a leading technique for large-scale, controlled synthesis of a wide range of nanostructured materials. In this work, ALD of Fe$_2$O$_3$ is demonstrated using FeCl$_3$ and H$_2$O precursors at growth temperatures between $200-350^{\circ}$C. Self-limiting growth of Fe$_2$O$_3$ is observed with a growth rate of $\sim0.06$ nm/cycle. As-deposited, films are nanocrystalline with low Cl impurities and a mixture of $\alpha$- and $\gamma$-Fe$_2$O$_3$. Post-deposition annealing in O$_2$ leads to phase-pure hematite with increased out-of-plane grain size. Photoelectrochemical measurements under simulated solar illumination reveal high photoactivity toward water oxidation in both as-deposited and post-annealed films. Planar films deposited at low temperature (235$^{\circ}$C) exhibit remarkably high photocurrent densities $\sim0.71$ mA/cm$^{2}$ at 1.53 V vs. the reversible hydrogen electrode (RHE) without further processing. Films annealed in air at 500$^{\circ}$C show current densities of up to 0.84 mA/cm$^{2}$ (1.53V vs. RHE).